android - SupportFragmentManager 的 findFragmentById 始终返回 null

Question

我正在尝试使用 findFragmentById() 获取 fragment 引用，但它总是返回空对象。我使用了 FragmentPagerAdapter 并附加了一个 OnPageChangeListener 这样每当我滚动到一个特定的 Fragment 时，比如 FragmentB，FragmentB< 的一个方法 应该被解雇。

我有以下代码:

mPager.addOnPageChangeListener(new OnPageChangeListener() {
    @Override
    public void onPageSelected(int position) {
        if (position == 1) {
            FragmentB fb = (FragmentB) getSupportFragmentManager().findFragmentById(R.id.fragment_b);

            fb.refreshList(); 
            //fb is always null

            Toast.makeText(MainActivity.this,"Scrolled", Toast.LENGTH_SHORT).show();                                                                         
            //Toast is working fine
        }
    }

    @Override
    public void onPageScrolled(int arg0, float arg1, int arg2){}

    @Override
    public void onPageScrollStateChanged(int arg0){}
});

Answer 1

只需这样做:

mPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        PagerAdapter pagerAdapter = (PagerAdapter) mPager.getAdapter();
        if (position == 1) {
            FragmentB fragmentB = (FragmentB)pagerAdapter.getItem(position);
            fragmentB.refreshList();
        }
    }

    @Override
    public void onPageSelected(int position) {}

    @Override
    public void onPageScrollStateChanged(int state) {}
});

findFragmentById的问题不打算这样用:

Finds a fragment that was identified by the given id either when inflated from XML or as the container ID when added in a transaction. This first searches through fragments that are currently added to the manager's activity; if no such fragment is found, then all fragments currently on the back stack associated with this ID are searched.

在使用 时，您应该调用它来获取 fragment 容器中的哪个 fragment (例如，如果您正在使用 Navigation Drawer 并且想知道哪个 fragment 在视口(viewport)中) ViewPager，托管多个 fragment 。

希望对您有所帮助。